I actually just got married a few days ago in an Applebee's and I can answer this!

Tax reasons, legal benefits, and insurance purposes. That's literally it. Oh, plus "wife" rolls of the tongue better than "girlfriend" or "fiance," and I don't have to keep explaining why we haven't gotten married yet despite living together for 2 years now. Both my wife and I were perfectly content living the rest of our lives unmarried as life partners. To us it's just a piece of paper. But with all of the financial and legal benefits, we both decided it's much more worth it to just get married, since as we had agreed, it's still just a piece of paper, and signing said paper would be financially and legally beneficial so might as well just do it.

So yeah, we got married at Applebee's the other day on our 3 year anniversary (we really wanted to keep that date as our anniversary too) where we had dinner with 4 of our closest friends and had them officiate/witness our marriage right in front of our boneless wings. We do plan on having an actual, formal ceremony down the road and inviting all of our family/friends, but for now we're happy with just signing the paperwork and reaping the benefits.

The big thing for me that really pushed me towards wanting to get married is when my wife's close relative died. I took some time off work to take care of her, but because we weren't married I had to take vacation time instead of bereavement leave, which I would have qualified for if we were married. In my opinion it's quite frankly bullshit how many tangible benefits marriage entails, I think it should just be an entirely personal affair, but unfortunately that's just not how the world works.